There is a great industrial need for large-area solid-state light sources for a series of applications, predominantly in the field of display elements, VDU technology and lighting engineering. The demands made of these light sources can at present not be fully satisfactorily met by any of the existing technologies.
As an alternative to conventional display and lighting elements, for example incandescent lamps, gas-discharge lamps and non-self-illuminating liquid crystal display elements, use has been made for some time of electroluminescence (EL) materials and devices such as light-emitting diodes (LEDs).
Apart from inorganic electroluminescence materials and devices, low molecular weight, organic electroluminescence materials and devices have been known for about 30 years (see, for example, U.S. Pat. No. 3,172,862). However, until recently such devices were greatly restricted in their practical usability.
EP-B 0 423 283 and EP-B 0 443 861 describe electroluminescence devices comprising a film of a conjugated polymer as light-emitting layer (semiconductor layer). Such devices offer numerous advantages such as the opportunity of producing large-area, flexible displays simply and inexpensively. In contrast to liquid crystal displays, electroluminescence displays are self-illuminating and therefore require no additional backward lighting source.
A typical device as described in EP-B 0 423 283 comprises a light-emitting layer in the form of a thin, dense polymer film (semiconductor layer) comprising at least one conjugated polymer. A first contact layer is in contact with a first surface of the semiconductor layer and a second contact layer is in contact with a further surface of the semiconductor layer. The polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers so that when an electric field is applied between the two contact layers, charge carriers are introduced into the semiconductor layer, with one contact layer becoming positive relative to the other, and the semiconductor layer emits radiation. The polymers used in such devices are conjugated. For the purposes of the present invention, a conjugated polymer is a polymer which has a delocalized electron system along the main chain. The delocalized electron system gives the polymer semiconducting properties and makes it able to transport positive and/or negative charge carriers with high mobility.
EP-B 0 423 283 and EP-B 0 443 861 describe poly(p-phenylene-vinylene) which has been modified on the aromatic ring by means of alkyl, alkoxy, halogen or nitro substitutents as polymeric material for the light-emitting layer. Such polymers have since been examined in a large number of studies and dialkoxy-substituted PPVs in particular have al ready be en optimized to a great extent in the direction of readiness for applications (cf. for ex ample, J. Salbeck, Ber. Bunsenges. Phys. Chem. 1996, 100, 1667). However, the development of such polymers can in no way be regarded as concluded. Thus, inter alia, improvements are still necessary in respect of operating life, stability and also the achievable color. With regard to the last point, the furthest developed class of polymers mentioned above, dialkoxy-PPV, is suitable only for the emission of orange-red light.
It is therefore an object of the present invention to provide new electroluminescence materials which, when used in lighting or display devices, is suitable for improving the property profile of these devices.